The present invention relates to a fixing device in which a recording material carrying a toner image is heated and is pressed between a heating roller and a pressure roller so that fixing is achieved, and an image forming apparatus including the same, and in particular, a fixing device wherein an external heating roller is provided which is in contact with the surface of the heating roller, and an image forming apparatus including the same.
In an image recording apparatus employing an electro-photographic method, such as a copying machine, a facsimile device, and a laser printer, widely used is a fixing device wherein toner images are formed on a photoconductor, and after the toner images are transferred onto a recording sheet, the sheet is conveyed between a heating roller, which houses a heat source in a cored cylinder and generates heat through an elastic layer which is formed on the outer circumferential surface of the cored cylinder, and a pressure roller which comes in contact with the heating roller, and thereby the toner images are permanently fixed onto the recording sheet by heat and pressure.
Recently, due to the increased speed of image recording apparatus, feeding rate of the recording sheets also has become higher, and further, due to colorization of image and improvement of image quality, a rubber layer, having lower heat conductivity and being normally used for the elastic layer of the heating roller, is required to become thicker, and therefore, it is necessary that the heat source mounted in the cored cylinder must supply an increased amount of heat.
However, when heat from the heat source in the cored cylinder is increased excessively, the elastic rubber layer is damaged by the heat, and the durability of the elastic rubber layer is deteriorated, whereby a problem results in that the elastic layer is unglued from the cored cylinder.
In order to overcome these problems, proposed are a fixing device and an image recording apparatus using the fixing device, providing an auxiliary heating roller which comes into pressure-contact with the heating roller, and heats the surface of the heating roller from the outside (Patent Documents 1 and 2), or a fixing device and an image recording apparatus, providing an auxiliary heating roller which heats the surface of the pressure roller from the outside (Patent Document 3).
Patent Document 1: Japanese Tokkai 2001-22214
Patent Document 2: Japanese Tokkai 2003-280435
Patent Document 3: Japanese Tokkai 2002-49265
However, in both of these technologies disclosed in Patent Documents 1 and 2, since a pressure roller is a hard roller, though high image quality such as halftone is required, reproductivity of the image is not sufficient. Further, in technology disclosed in Patent Document 3, a heat source is stored in a first fixing roller (heating roller), it takes long times to heat up the surface of the elastic layer to the level of fixing temperature, and if the heating capacity of the heat source is increased too large, the elastic layer made of a rubber is affected by the heat deterioration, which is a problem.